The Portable Gas Analyzer Based on the Spectrum

With the rapid development of economic and people living standard unceasing improvement the popularity of the development of heavy industry and transportation, with a large number of harmful substances into the environment atmosphere, changed the composition of normal air, worsen air quality[1]. Health problems, which caused by the atmospheric pollution has caused governments and people of great importance to it. Therefore, monitoring SO₂ and NO_x become one of the important subject of environmental protection[2]. For the accurate monitoring of the real-time quality, ecological environment and pollution to the environment, to provide accurate basis for the environmental protection departments at various levels supervision, management and environment decision, urgent need a large number of modern environmental monitoring instruments.

Through to the present situation of flue gas analyzer in the domestic and foreign research and analysis, portable flue gas on the domestic market at present the main monitoring method is electrochemical analysis instrument, infrared gas analyzer and differential absorption spectrum. Electrochemical analysis instrument each sensor can only be measured a chemical composition in flue gas, if need to measure six ingredients you need six sensors. The chemical battery principle led to zero drift in the development of the system and cross interference, sensor lifetime is also difficult to solve the problem[3]. There is no absorption peak to NO₂ in the infrared spectrum analyzer, cannot monitor NO₂, but can only detect the NO content, through the assumptions to NO₂ concentrations. At the same time as infrared spectrum analyzer adopts wheel filter method, each time only one gas measurement, can not measure a variety of gases at the same time[4]. Differential absorption measured gas to uv and visible light waves, on the basis of the differential absorption spectrum by the strength of the differential absorption spectrum inversion gas concentrations, in the traditional algorithm of differential absorption, the application of least square method of gas concentration for calculating optimal estimation equation is complex, and easy in pathological conditions[5]. Accordingly based on Lambert Beer law and absorbance of dual stack, a mutually recursive iteration is presented more gas concentration decoding algorithm, the process of the algorithm is presented, and verify the effectiveness of the algorithm. This system using the Ocean Optic micro spectrometer combining with embedded technology company has developed a set of portable flue gas analyzer[6]. This instrument is to overcome the disadvantages of the electrochemical sensor, had been achieved and sensor life problems, and at the same time for accurate measurement of gases and no cross interference.

Portable flue gas analyzer system structure consists of three parts, part data collection, data processing part and the human-computer interaction part. Data acquisition part by sampling probe, flue gas sampling pump, an ultraviolet light and miniature spectrum analyzer, collecting gas mainly accomplished by ultraviolet light and miniature spectrum analyzer spectrum before and after the data is converted into digital signal transmitted to the data processing part. Data processing part of ARM microcontroller CPU core module and peripheral circuit, main is to electrical signals were collected by using corresponding algorithm to calculate the gas concentration of all kinds of gas in the flue gas composition. The human-computer interaction part consists of LCD display and keyboard, mainly to complete the real-time display of gas concentration, and through the keyboard to control the storage of data and related parameters Settings.

Circular iteration method using various gases in 190 ~ 290 nm band characteristic absorption peak, combined with the absorbance of dual stack, a feature in some gas absorption point assuming that other gas no absorption, launched the initial concentration of the gas, then switch to another characteristic absorption point, the gas absorbed photon number from the measured total absorbed photon number subtracting, get another gas initial concentration, and so on to get the initial concentration of each gas. Then return to the first, the characteristics of the gas absorption point from the total number of photons absorbed minus every other gas absorption of the photon number, again to get the first gas concentration of an iteration, and so on to get other gas concentrations of an iteration, the repeated iteration until the concentration difference between two times smaller than a certain value, the concentration of each gas to the gas concentration accurately. Algorithm steps are as follows.

First step: to solve the first gas in the gas mixture of initial concentration C₁₀, and selects a characteristic wavelength λ₁, under the specific wavelength of a gas in the gas mixture has obvious characteristic absorption peak, and other gas absorption peak is small at this point, is to read the value of the absorbed photon number S_λ1 and solve Rλ−DλSλ−Dλ is obtained by absorbance look-up table and the concentration of the gas as the initial concentration of mixed gas in the gas.

Step 2: solving the second gas in the gas mixture of initial concentration: select the second characteristic wavelength in the wavelength of λ₂ second kind of gas has obvious absorption peak, and other gas absorption peak is weak, then read the absorbed photon number S_λ2 considered under the band is only two kinds of mixed gas, according to formula (7) to calculate the second gas absorbance, look-up table against solving calculation, the second gas concentrations, as the second kind of initial concentration C₂₀ of gases.

Step 3: calculating of other gases in the mixed gases, the initial concentration: the mth characteristic wavelengths selected λ_m, the wavelength of the first m gas has obvious absorption peak, read the wavelength of absorption of the photon number S_λm by formula (7) and look-up table type inversion calculation of this kind of gas concentration as the initial concentration C_m0 of the gas.

Step 4: iterative inversion to calculate the first gas concentration of first order recursive: will the desires of all kinds of gas concentration in formula (7), again read λ₁ of S_λ1 reverse the first gas concentration C₁₁ of first order recursive.

Step 5: repeat the second and third step to calculate gas m first order recursive concentration C_1m.

Step 6: calculation of gas concentration of the adjacent two iterative error; Calculating first-order differential iteration of each gas is shown in the following formula,(9)Δ1=|Com−Cm1|

Select Δ₁ maximum of first-order differential iteration as the iteration error, i.e(10)Δ1=max{Δm1,m=1,2,…,M}

Step 7: repeat the fourth, fifth and sixth step, until the iteration error is less than the given value namely.(11)Δ1=max{Δm1,m=1,2,…,M}

Step 8: termination of the algorithm, it will be the last time the concentration as a final concentration of gases.

The obtained by experiments, So₂No₂, the NO and NH₃ four gas absorption lines as shown in figure 1, in the range of 190 nm to 190 nm, four types of gas are interfered with each other. At wavelength of 1 (273.33nnl) So₂ and No₂ are absorbed. Also at wavelength of 2 (231.33nm) So₂ and No₂ are absorbed. Only at wavelength of 3 (225.88nm) NO, So₂ and No₂ are absorbed. At wavelength of 4 (208.23nm) NH₃, No₂ and So₂ are absorbed. The So₂ and No₂ have absorbed at each wavelength point, For NH₃ and NO, and, as long as the concentration of So₂ and No₂ first came out, and then the corresponding concentration on wavelength 3 or 4 of the absorbance minus the can get their absorbance. So should first of all, the concentration of So₂ and No₂, then the concentration of NH₃ and NO.

Figure 1.

A.

Calculate the concentration of So₂ and No₂

For the concentration of NH₃ and NO are the premise of know the concentration of So₂ and No₂. However, So₂ and No₂ throughout the working wave band are interfered with each other, need an algorithm to eliminate their interference. Here, on the basis of the look-up table using loop iteration method and calculation method to determine the concentration of So₂ and No₂.

As shown in figure 2, At the wavelength of 1 and 2 at the So₂ and No₂ were absorbed, they differ greatly in the absorption intensity at two wavelengths, the absorbance of the absorbance of So₂ and No₂ at 273.33nm is far less than the same concentration of No₂ in 231.33nm, and the So₂ absorbance at 231.33nm is far less than the same concentration of So₂ in the absorbance at 273.33nm. Then we can be absorbed in 231.33nm No₂ called the main absorption of No₂, No₂ in the absorption of 273.33nm is called So₂ interference, So₂ in the absorption at 273.33nm is the main absorption, in 231.33 absorption interference on No₂ main absorption. Iterative method is through circulation calculation to gradually eliminate the interference of No₂ on So₂, the true concentration approaching So₂ and No₂.

Figure 2.

Cyclic iteration method to overcome the conflict there understand equations caused no solution of the problem, and this method can be easily implemented by software programming. Statistical chart concentrations of So₂ and No₂ calculated with iteration number as shown in the figure 3-4. The abscissa is the number of iterations, the longitudinal concentration coordinates So₂ and No₂ (ppm).

Figure 3.

Figure 4.

According to the running results and statistics can be seen in figure, the process of iteration is a successive approximation process, the calculation results of So₂ with the increase in the number of iterations decreases gradually, and tends to be stable, but No₂ is exactly the opposite, results with increasing iteration times increase. Cyclic iterative method in two iterations later effect is not obvious, which is to say as long as two iterations. This can be calculated through No₂ and So₂ concentrations.

Main harmful components for atmospheric environmental pollution monitoring requirements, using uv wavelength grating type continuous frequency measuring method, precision is put forward to solve the various harmful ingredient concentration of recursive iteration fast inversion algorithm, and validates the effectiveness of the algorithm. Portable flue gas analyzer based on this algorithm is based on embedded technology, sensor based on micro spectrometer data collection, using uv light source through the spectrum analysis method analysis of flue gas concentration, achieved through the use of a miniature spectrum analyzer to a variety of gas composition at the same time for the purpose of accurate measurement. The product has compact structure, high measuring accuracy, strong anti-interference, high sensitivity etc, has a broad application prospect and popularization value.